The Influence of the Hofmeister Bias and the Stability and Speciation of Chloridolanthanates on Their Extraction from Chloride Media

<p>The possibility of recovering rare earth elements from solutions containing their chloridometalate anions [LnCl<i>_x</i>]^{(<i>x</i>−3)−} via the process: LnCl<i>_x</i>^{(<i>x</i>−3)−} + (<i>x</i> − 3)<i>L</i>_org + (<i>x</i>–3)H⁺ ⇌ [(LH)<i>_x</i>₋₃LnCl<i>_x</i>]_org has been tested using 2-(1,3-bis(hexylamino)-1,3-dioxopropan-2-yl)-4,6-di-<i>tert</i>-butylpyridine (PMA), tri-<i>n-</i>butylphosphate (TBP), and tri-<i>n</i>-octylamine (TOA), which are known to be strong extractants for transition metal chloridometalates. While DFT calculations indicate that the formation of the neutral assembly [(PMAH)₃LaCl₆] in the gas phase is favorable, no uptake of La(III) from 6 M HCl by toluene solutions of PMA (or of TBP or TOA) was observed in solvent extraction experiments. Successful uptake of the [PtCl₆]²⁻ dianion by PMA and the failure to extract the [IrCl₆]³⁻ trianion under the same conditions indicate that the higher hydration energy of the latter makes transfer to the toluene solution less favorable and that this militates against extraction of La(III) chlorido complexes carrying charges of −3 or larger in which all the inner-sphere water molecules have been replaced. Computational results confirm literature observations that, in contrast to transition metal trications, formation of REE metalate anions such as [LnCl<i>_x</i>]^{(<i>x</i>−3)−} is not very favorable, particularly so for chloride, compared with nitrato or sulfato systems. Also, they indicate that the formation of <i>outer-sphere</i> assemblies such as {[La(H₂O)₉]·<i>x</i>Cl} in which water ligands are retained in the inner sphere, H-bonded to anions, is more stable than <i>inner-sphere</i> complexes containing an equivalent number of anions. The high level of hydration of such species disfavors their transfer into nonpolar water-immiscible solvents. It is unlikely that recovery of [LnCl<i>_x</i>]^{(<i>x</i>−3)−} from acidic solutions can be achieved efficiently using currently available anion exchange extractants operating in a “pH-swing” process. Receptors giving very high binding energies to chloridolanthanates will be needed to offset the high dehydration energies required.</p

Anion receptor design; exploiting outer sphere coordination chemistry to obtain high selectivity for chloridometalates over chloride.

High anion selectivity for PtCl₆^2– over Cl^– is shown by a series of amidoamines, R¹R²NCOCH₂CH₂NR³R⁴ (L1 with R¹ = R⁴ = benzyl and R² = R³ = phenyl and L3 with R¹ = H, R² = 2-ethylhexyl, R³ = phenyl and R⁴ = methyl), and amidoethers, R¹R²NCOCH₂CH₂OR³ (L5 with R¹ = H, R² = 2-ethylhexyl and R³ = phenyl), which provide receptor sites which extract PtCl₆^2– preferentially over Cl^– in extractions from 6 M HCl solutions. The amidoether receptor L5 was found to be a much weaker extractant for PtCl₆^2– than its amidoamine analogues. Density functional theory calculations indicate that this is due to the difficulty in protonating the amidoether to generate a cationic receptor, LH⁺, rather than the latter showing weaker binding to PtCl₆^2–. The most stable forms of the receptors, LH⁺, contain a tautomer in which the added proton forms an intramolecular hydrogen bond to the amide oxygen atom to give a six-membered proton chelate. Dispersion-corrected DFT calculations appear to suggest a switch in ligand conformation for the amidoamine ligands to an open tautomer state in the complex, such that the cationic N–H or O–H groups are also readily available to form hydrogen bonds to the PtCl₆^2– ion, in addition to the array of polarized C–H bonds. The predicted difference in energies between the proton chelate and nonchelated tautomer states for L1 is small, however, and the former is found in the X-ray crystal structure of the assembly [(L1H)₂PtCl₆]. The DFT calculations and the X-ray structure indicate that all LH⁺ receptors present an array of polarized C–H groups to the large, charge diffuse PtCl₆^2– anion resulting in high selectivity of extraction of PtCl₆^2– over the large excess of chloride